The programmed control of viral gene expression has often served as a simplified model of the processes occurring in cells. Bacterial viral systems have been particularly useful in this respect, and have generated many concepts that have direct bearing on such eukaryotic processes as cell differentiation, loss of essential cell functions, and carcinogenesis. Regulation of bacteriophage T7 gene expression occurs at both the level of transcription and translation. Mutants of this phage which are defective in control of host translation have been identified in this laboratory. The effects of these mutations on the synthesis of host and viral enzymes, both in vivo and in a cell free translation system will be analyzed. These mutants will also be useful in understanding the manner in which the virus regulates the synthesis of host mRNA. Poxvirus gene expression is also subject to a variety of controls. By examining the 5'-termini of RNA synthesized in vitro by the DNA-dependent RNA polymerase of this virus it will be possible to determine whether the "delayed early" subclass of early mRNA arises by elimination of transcriptional termination barriers, or by recognition of a new set of promotors. Further, the possibility that post-transcriptional cleavage of early RNA is involved in the efficiency of translation of these messengers will be examined.